Abstract: This disclosure describes a fraudulent website detection system that provides a framework for locally detecting fraudulent websites on a client device. For example, using a local lightweight machine learning model, the fraudulent website detection system can detect and respond to fraudulent websites in real time. In some examples, the fraudulent website detection system is integrated into a web browser to promptly identify fraudulent websites. Moreover, the fraudulent website detection system, operating on multiple client devices, can collaborate with an online threat detection system to quickly notify other client devices about fraudulent websites and to utilize aggregated reports to improve the lightweight machine learning model.